Re: [Xen-devel] [PATCH v4 15/18] xen/mem_sharing: VM forking

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Jan 09, 2020 at 06:41:12AM -0700, Tamas K Lengyel wrote:
> On Thu, Jan 9, 2020 at 3:29 AM Julien Grall <julien@xxxxxxx> wrote:
> >
> > Hi Tamas,
> >
> > On 08/01/2020 17:14, Tamas K Lengyel wrote:
> > > +static int mem_sharing_fork(struct domain *d, struct domain *cd)
> > > +{
> > > +    int rc;
> > > +
> > > +    if ( !d->controller_pause_count &&
> > > +         (rc = domain_pause_by_systemcontroller(d)) )
> >
> > AFAIU, the parent domain will be paused if it wasn't paused before and
> > this will not be unpaused by the same hypercall. Right?
> 
> Yes, it needs to remain paused as long as there are forks active from
> it. Afterwards it can be unpaused.

If you want the parent domain to remain paused for as long as the
forks are active, shouldn't each fork increment the pause count on
creation and decrement it when the fork is destroyed?

How can you assure no other operation or entity has incremented
controller_pause_count temporary and is likely to decrement it at some
point while forks are still active?

Thanks, Roger.

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